[:en]“That sun is trying to kill us” is something I hear every other day from my wife. Growing up and settling in the Midwest of the USA, she is used to the beating heat from our local star. I remember a particular summer when the consecutive number of days over 100F (~38C) was well over 60.

As you can imagine this post is about the sun. (By which, I mean the star closest to us, but similar principles would apply to other solar systems). The emphasis will be made on understanding what this energy is, and how we can harness it.

First, let’s discuss solar energy in general. As its name suggests, this type of energy comes from the sun. (Solaris means sun in Latin and is where our word solar comes from). So far, so good. Now, even though “radiation” gets a bad reputation, this is actually how the heat and energy from our star reaches us. The radiation is produced by nuclear reactions in our sun’s core. Two hydrogen atoms get fused together to form one helium atom. The chemical reaction releases heat and light. And all of this is occurring inside the sun 93 million miles away in space. The light and heat travel through space. Then some of that energy, in the form of radiation, reaches us here on Earth.

Now that we know what energy solar energy is and where it comes from, let’s briefly discuss the processes we currently have to capture this energy and what uses we can make of it.

There are primarily two types of sun power harnessing systems:

Solar panels

Concentrated Solar Power (also known as CSP)

Solar panels are typically photovoltaic (PV) which means that they will convert photon energy (photo) into electricity (voltaic). When you think of such technology the roof of houses and office buildings (PV panels – comprised of several PV cells) is usually the first example to come to mind. But, don’t forget the small solar cells used to power your calculator (PC cell), or the much larger installations on the side of the highway (PV arrays – comprised of multiple PV panels). After capturing this solar energy, you can either use it for your personal needs, or in some cases you can sell it back to the grid. Note: Amazon recently completed its 17th rooftop solar project by installing a 1.1 MW array on its Las Vegas fulfillment center (https://www.renewableenergyworld.com/articles/2018/05/amazon-s-onsite-solar-just-went-up-a-notch.html).Another way solar panels work for domestic application is to circulate a liquid through the panels to heat the home (air heating, water heating, and so on).

CSP use a different technology altogether. Fields of mirrors (that rotate with the sun) are used to concentrate the energy from the sun into what is called a “black body”. In heat transfer terms, this refers to something that has a high thermal coefficient (emissivity) and typically sits at the top of a tower. If you have ever used a magnifying glass to concentrate solar energy on some dry twigs to start a fire, you have seen how effective this approach can be.

The previous blog post of this series mentioned that both nuclear and solar sources were considered clean energies with solar being renewable while our sun still shines. What makes it clean exactly? I am glad you asked! (I know you did not, but let’s pretend you did.) To quote my last post, clean energies are defined as “energies that do not pollute the atmosphere when used.” With solar energy, the process of energy creation is indeed harmless to the surrounding. The environmental impact of the systems to manufacture items needed to capture the solar energy and recycling/disposing of waste products from that process may pollute. Some will argue that solar arrays can be a visual pollution, but that objective opinion does not make solar a “dirty” energy since gathering the energy neither produce pollutants nor emits carbon dioxide.

Solar is great, but it definitely does have some limiting factors. Indeed, to get the most out of the sun’s power, we need to have a few things:

Intensity: The hotter and the brighter the sun is, the easier it will be to harness solar power. During a recent trip to Nicaragua, the UV index reached “Extreme”. That intensity will work just fine to use solar energy.

Duration: If you only have sunlight for a few hours such as during the wintery months in Northern Sweden, then you are out of luck that time of year for harnessing solar energy). Indeed, the amount of sunlight varies depending on location, time of day, season of the year, and weather conditions so you could very well be in a very sunny region sitting under a seemingly paralyzed cloud watching your solar panel do nothing for you.

Another limitation is due to the fact that the amount of sunlight reaching a square foot of the earth’s surface is relatively small, so a large surface area is necessary to absorb or collect a useful amount of energy. The further you are from the equator and the less area is receiving the same solar energy which leads to the main trend in the heat map below.

Figure 2 World solar heat map – NASA

Let’s take a quick look at the rovers that are exploring the surface of planet Mars. The first few rovers mainly derived their energy from the sun. But the accumulation of dust, – and sometimes being caught in sand storms bigger than North America -, combined with the limited number of hours of sunlight available during Martian winters, solar power on Mars turned out less reliable than we hoped. This is why Curiosity, the newest rover to be sent which recently found carbon traces in a hole it drilled, uses a nuclear generator. That is a story for a different post.

We have seen: what solar energy is; where is comes from; where to find it; how to harness it; and what we can use it for. With solar energy being the world’s fastest growing source of power, its cost has dropped significantly since solar panels were first introduced to the market (https://www.businessinsider.com/solar-power-cost-decrease-2018-5?r=US&IR=T) and its low impact on the environment makes it a very attractive clean energy at both smaller and larger scales. All in all, I for one am happy to be getting free electricity for the lamps on my balcony.

“That sun is trying to kill us” is something I hear every other day from my wife but with all the potential that this energy source has what resonates in my mind is “that sun is hopefully going to save us”.